Housing First is a model for serving homeless individuals with dual diagnoses of substance use disorder and serious mental illness. While model has been associated with higher substance abuse treatment access for clients, it has been recognized that stronger definitions of its essential elements are needed before further investigation into the association between Housing First programming and substance abuse treatment can be carried out. The following application requests support for doctoral dissertation research that aims to: (1) define the essential components of the Housing First model;(2) develop a fidelity index for Housing First programming;(3) test the finalized fidelity index for reliability and validity;and (4) assess the degree to which fidelity predicts improved substance abuse treatment access for clients. The study seeks to address these aims through activities carried out in three phases. In the first phase interviews with staff and clients from Housing First organizations will define the core essential elements of the model. In the second phase, interviews with Housing First experts will be conducted to create a finalized fidelity index. The final phase will assess the model by correlating agency fidelity scores with housing retention and client treatment access data from housing organizations. PUBLIC HEALTH RELEVANCE: The Housing First model is based on human rights and public health principles. In relation to substance abuse, Housing First seeks to assist individuals to engage in treatment on their own terms as a means to obtaining stronger treatment outcomes. This project seeks to advance research on the Housing First model by developing a fidelity index that will allow for stronger measurement of Housing First programming for implementation and evaluation purposes.